The present invention relates to a process for producing an ethylenic oligomer. More particularly, the present invention is concerned with a process for producing an ethylenic oligomer which is capable of producing a high quality ethylenic oligomer having enhanced purity, and in which a Ziegler based catalyst can maintain a high catalytic activity, when producing an ethylenic oligomer which is useful as a starting material for high molecular polymers, plasticizers, surfactants and the like by the use of the foregoing catalyst.
An ethylenic oligomer is a useful substance which is widely used as a starting monomer material for olefin polymers, as a comonomer for a variety of high molecular polymers, as a starting material for plasticizers, surfactants and the like. Usually, the ethylenic oligomer is produced by oligomerizing ethylene as the starting material by the use of a Ziegler based catalyst. The production process generally comprises the step of oligomerization reaction, the step of recovering unreacted ethylene, the step of deactivating the catalyst and deashing and the step of fractionating the solvent used therein and ethylenic oligomer.
A production process in which a Ziegler based catalyst is used as a catalyst for oligomerization reaction involves a problem in that a large amount of water, when being mixed in the reaction system, brings about partial deactivation of the catalyst, which as a result, causes deterioration of product quality such as the formation of organochlorine components upon ethylene oligomerization.
Such being the case, the process for producing an ethylenic oligomer by using a Ziegler based catalyst involves a fear of deteriorating the catalytic activity due to water present in the reaction system, particularly the water introduced from a reaction solvent into the reaction system. In particular, in the case of recovering unreacted ethylene from the oligomerization product, deactivating the catalyst and deashing, thereafter recycling the organic solvent separated by distillation through the reaction system and reusing it as a reaction solvent, there has been a fear of a large amount of water being mixed in the reaction system during the operation. Accordingly, it has been desired to decrease the amount of water mixed in the reaction system, particularly the amount of water in the reaction solvent to be recycled and at the same time, to maintain the catalytic activity at a highest possible level.
In the method comprising the above-mentioned process, in the case of recycling the organic solvent finally obtained in a fractionating system through the oligomerization reaction system for the purpose of reuse, considerable amounts of resultant olefins as ethylenic oligomers are mixed in the organic solvent. In general, the ethylenic oligomer produced by the foreging process contains impurities such as paraffin, internal olefin and branched olefin, which impurities cause marked deterioration in the quality of a polyethylene resin and the like as the final products. In particular, it has been proved that in the case of recycling an organic solvent in the oligomerization reaction system for use as a reaction solvent, the foregoing unfavorable result is made remarkable by the existence of an olefin which is other than ethylene and is contained in the organic solvent. Moreover the catalyst, which is formulated in the presence of a solvent, brings about the possibility of causing deterioration in catalytic activity or clogging in a catalyst feed line due to olefin polymerization.
That is to say, the present invention consists in providing a method for maintaining a catalytic activity at a high level in a process for producing an ethylenic oligomer which employs a Ziegler based catalyst and recyclingly uses a reaction solvent.
Moreover, the present invention consists in providing a method capable of affording a highly pure ethylenic oligomer free from an impurity in a process for producing an ethylenic oligomer which employs a Ziegler based catalyst and recyclingly uses a reaction solvent.
In view of the above-mentioned subject, intensive extensive research and investigation were accumulated by the present inventors. As a result, it has been found that the objects of the present invention can be achieved by reducing water present in the reaction system, particularly the water introduced from a reaction solvent into the reaction system, especially from an organic reaction solvent to be recyclingly used thereinto. Further it has been found that the objects of the present invention can be achieved by reducing olefinic components that are other than ethylene and are present in the reaction system, particularly the aforesaid olefinic components introduced from a reaction solvent into the reaction system, especially from an organic reaction solvent to be recyclingly used thereinto. Thus the present invention has been accomplished on the basis of the foregoing findings and information.
Specifically, the present invention provides a process for producing an ethylenic oligomer which comprises subjecting ethylene to oligomerization reaction in an organic solvent in the presence of a Ziegler based catalyst, and recyclingly using for the oligomerization reaction, the organic solvent separated by distillng the resultant oligomerization reaction product, wherein the water concentration in the oligomerization reaction system is at most 8 ppm by weight, in particular the water concentration in the organic solvent to be recycled in the oligomerization reaction system is at most 8 ppm by weight (first aspect of the present application).
In addition, the present invention provides a process for producing an ethylenic oligomer which comprises subjecting ethylene to oligomerization reaction in an organic solvent in the presence of a Ziegler based catalyst, and recyclingly using for the oligomerization reaction, the organic solvent separated by distillng the resultant oligomerization reaction product, wherein the concentration of olefins having at least three carbon atoms in the organic solvent to be recycled in the oligomerization reaction system is at most 2% by weight (second aspect of the present application).